The disclosures of Japanese Patent Applications No. 2000-078986 filed on Mar. 21, 2000 and No. 2000-107101 filed on Apr. 7, 2000, each including the specification, drawings and abstract, are incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates in general to an internal combustion engine installed on a vehicle, such as an automotive vehicle, and more particularly to an internal combustion engine which includes a variable valve control system capable of changing the opening and closing timing and/or a lift of each of intake and exhaust valves mounted in the engine.
2. Description of Related Art
Internal combustion engines installed on automobiles have recently been desired to purify exhaust gases emitted from the engines of harmful gaseous components such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained therein, before the exhaust gases are released into the atmosphere.
In order to meet the above-indicated demand, a known technique provides a three-way catalyst in the exhaust passage of the internal combustion engine. The three-way catalyst is arranged to reduce or remove hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in exhaust gases that have a predetermined air/fuel ratio equal to or close to the stoichiometric air/fuel ratio. For controlling the air/fuel ratio of the exhaust gases to which the three-way catalyst is exposed, the air/fuel ratio of an air-fuel mixture to be introduced into the engine is controlled in a feedback fashion, so that the exhaust gases emitted from the engine have the predetermined air-fuel ratio, enabling the three-way catalyst to purify the exhaust gases of desired harmful gaseous components.
On the other hand, internal combustion engines of lean-burn type have been developed for use on automobiles, in an effort to reduce the required amount of consumption of the fuel by the engine. The lean-burn type engine is capable of burning a fuel-lean air-fuel mixture the air/fuel ratio of which is higher than the stoichiometric value.
The exhaust gases to be emitted from such lean-burn type internal combustion engines have a relatively high air/fuel ratio, and contain a relatively small amount of reducing components such as hydrocarbon (HC), so that nitrogen oxides (NOx) contained in the exhaust gases can not be sufficiently reduced or removed by the three-way catalyst alone.
In view of the above situation, it has been proposed to use an occlusion-reduction type NOx catalyst disposed in the exhaust passage of the internal combustion engine. The occlusion-reduction type NOx catalyst is adapted to occlude or absorb nitrogen oxides (NOx) contained in fuel-lean exhaust gases to which the NOx catalyst is exposed when the exhaust gases have a fuel-lean air/fuel ratio, and to release the occluded nitrogen oxides (NOx) when the exhaust gases have a stoichiometric or fuel-rich air/fuel ratio. The released nitrogen oxides (NOx) are reduced into nitrogen (N2) by reaction with the reducing agents or components such as hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gases.
With the NOx catalyst of the occlusion-reduction type disposed in the exhaust passage of the lean-burn internal combustion engine, nitrogen oxides (NOx) contained in the exhaust gases are occluded or absorbed in the NOx catalyst, when the fuel-lean exhaust gases are emitted as a result of combustion of a fuel-lean air-fuel mixture within the engine.
When the internal combustion engine is operated with a stoichiometric or fuel-rich air-fuel mixture having the stoichiometric air/fuel ratio or an air/fuel ratio lower than the stoichiometric value, and the exhaust gases emitted from the engine are stoichiometric or fuel-rich, the occlusion-reduction type NOx catalyst releases the occluded nitrogen oxides (NOx), and the released nitrogen oxides are reduced into nitrogen (N2).
The amount of nitrogen oxides (NOx) that can be occluded or stored in the occlusion-reduction type NOx catalyst is limited, and the capacity of the NOx catalyst to occlude nitrogen oxides (NOx) is saturated after a long period of operation of the engine with a fuel-lean air-fuel mixture, leading to a possibility of releasing the nitrogen oxides into the atmosphere without being reduced.
To avoid the above-indicated possibility, it has been practiced to perform so-called xe2x80x9crich spikexe2x80x9d control when the NOx occlusion capacity of the occlusion-reduction type NOx catalyst is saturated during the lean-burn operation of the engine. In the xe2x80x9crich spikexe2x80x9d control, the mode of operation of the engine is switched from the lean-burn mode to the rich-burn mode so that the NOx catalyst is exposed to fuel-rich exhaust gases having a relatively low air/fuel ratio.
However, merely increasing the amount of fuel injection into the engine undesirably causes a sudden increase of the output torque of the engine. To prevent this sudden increase, the fuel injection amount must be increased while the amount of air to be drawn into the engine is reduced. Accordingly, the rich spike control requires both of the fuel injector valve and the throttle valve to be controlled so as to increase the fuel injection amount while reducing the intake air amount.
The throttle valve is located some distance apart from the combustion chamber in each cylinder, and therefore the actual reduction of the air amount in the combustion chamber to a desired value requires a certain time after the moment at which the opening of the throttle valve was reduced. Thus, the switching of the engine operation mode from the lean-burn mode to the rich-burn mode requires a relatively long time due to a delayed control response of the throttle valve.
Further, the throttle valve and the fuel injector valve must be controlled again after the termination of the rich-spike control, in order to increase the air amount to be drawn into the cylinder of the engine and reduce the fuel amount to be injected into the cylinder. Like the switching of the engine operation upon initiation of the rich-spike control, the switching of the engine operation mode from the rich-burn mode to the lean-burn mode requires a relatively long time due to a delayed response of the throttle valve after the termination of the rich-spike control.
Thus, the rich-spike control requires a considerably long time due to the long periods of time required for switching the mode of operation of the engine operation between the lean-burn and rich-burn modes upon initiation and termination of the rich-spike control. This may cause undesired deterioration of the driveability of the vehicle and an unnecessary increase in the amount of consumption of the fuel.
Some of the lean-burn internal combustion engines such as diesel engines and lean-burn gasoline engines use the above-described NOx catalyst of the occlusion-reduction type or selective reduction type or other exhaust emission purifying device for purifying the exhaust gases emitted therefrom.
The selective reduction type NOx catalyst is a catalyst capable of reducing or decomposing nitrogen oxides (NOx) in an oxygen-rich atmosphere, in the presence of hydrocarbon (HC). For this selective reduction type NOx catalyst to be able to reduce or remove NOx, an appropriate amount of HC component is required. When the selective reduction type NOx catalyst is used to purify exhaust gases emitted from the above-indicated lean-burn internal combustion engines, the selective reduction type NOx catalyst needs to be supplied with the hydrocarbon (HC) component, since the amount of the HC component in the exhaust gases emitted during a normal lean-burn operation of the engine is extremely small. The supply of the HC component may be achieved by operating the engine with a fuel-rich or stoichiometric air-fuel mixture so that the exhaust gases emitted from the engine have the stoichiometric air/fuel ratio or an air/fuel ratio lower than the stoichiometric value.
As discussed above, the occlusion-reduction type NOx catalyst is adapted to occlude NOx when the emitted exhaust gases are fuel-lean, and release and reduce the occluded NOx when the oxygen concentration of the exhaust gases is reduced.
When the occlusion-reduction type NOx catalyst is used to purify exhaust gases to be emitted from the lean-burn internal combustion engines, NOx in the fuel-lean exhaust gases are occluded in the NOx catalyst during a normal lean-burn operation of the engine. The NOx cannot be occluded in the NOx catalyst after the NOx occlusion capacity of the catalyst is saturated as a result of the lean-burn operation of the engine for a long time. In this event, NOx is released into the atmosphere. To avoid this situation, it is necessary to cause the exhaust gases to be fuel-rich and to considerably reduce the oxygen concentration of the exhaust gases, at a suitable point of time before saturation of the NOx occlusion capacity of the NOx catalyst, in order to increase the amount of HC as a reducing agent, release the occluded NOx from the NOx catalyst and reduce the released NOx into N2. In this manner, the NOx catalyst may be able to recover the original NOx occlusion capacity.
Thus, the emission purifying device utilizing the lean-burn NOx catalyst requires the supply of hydrocarbon (HC) as the reducing agent for reducing and removing NOx, and therefore requires the exhaust gases to be stoichiometric or fuel-rich from time to time. JP-A-6-17225 discloses one example of a method of controlling the exhaust gases to be stoichiometric or fuel-rich, which utilizes an auxiliary or secondary injection of fuel into the engine.
The auxiliary fuel injection is a fuel injection into the engine cylinder following the primary fuel injection into the engine cylinder made for providing the desired output torque of the engine. The auxiliary fuel injection takes place during the expansion stroke or exhaust stroke.
However, a portion of the fuel injected into the cylinder in the secondary fuel injection for the purpose of controlling the air/fuel ratio of the exhaust gases may be burned in the cylinder, increasing the output torque of the engine by a small amount, depending upon the operating condition of the engine, so that the vehicle drive force is increased with some shock, resulting in undesirable deterioration of the vehicle driveability. In this respect, it is desired to develop any suitable method of controlling the air/fuel ratio of the exhaust gases (or increasing HC as the reducing agent), other than the auxiliary fuel injection.
For controlling the exhaust gases to be stoichiometric or fuel-rich, there is also known a method in which the air-fuel mixture introduced into the combustion chamber is controlled to have the stoichiometric air/fuel ratio or an air/fuel ratio lower than the stoichiometric value. If the air/fuel ratio of the air-fuel mixture is immediately changed from a value higher than the stoichiometric value to the stoichiometric or lower value, however, there may be a risk of misfiring of the engine due to a delayed mixing of the air and the fuel into the desired air-fuel mixture.
Accordingly, the air/fuel ratio of the air-fuel mixture must be sufficiently gradually changed from a lean level (that is higher than the stoichiometric value) to a rich level (that is lower than the stoichiometric value).
However, the gradual change of the air/fuel ratio requires a relatively long time until the desired stoichiometric or lower air/fuel ratio is reached. During this period of time, NOx is less likely to be reduced and removed by the selective reduction type NOx catalyst, or NOx releasing and reduction is less likely to be effected by the occlusion-reduction type NOx catalyst. In either of these two types of NOx catalysts, the NOx reducing/removing capability and the fuel economy of the engine are undesirably deteriorated.
It is therefore an object of the present invention to recover an NOx catalyst used in an internal combustion engine of a lean-burn type capable of burning an oxygen-rich air-fuel mixture, such that the NOx catalyst is placed, at an early opportunity, in a state suitable for reducing or removing selected gaseous components in exhaust gases emitted from the engine while the exhaust gases should be purified of the gaseous components, and such that the amount of consumption of the fuel by the engine is reduced.
To accomplish the above and/or other objects, one aspect of the present invention provides an apparatus comprising (a) an internal combustion engine of a lean-burn type capable of burning an oxygen-rich air-fuel mixture, which engine includes an intake valve and an exhaust valve for each of a plurality of cylinders thereof, (b) an NOx catalyst disposed in an exhaust passage of the internal combustion engine so as to remove nitrogen oxides contained in an exhaust gas emitted from the engine, (c) a variable valve control system capable of changing the opening and closing timing and/or a lift of at least one of the intake valve and the exhaust valve for each cylinder of the engine, and (d) a controller configured to control the variable valve control system to thereby control the exhaust gas to which the NOx catalyst is exposed, such that the controlled exhaust gas is suitable for removing a selected gaseous component from the NOx catalyst when the selected gaseous component should be removed.
In the apparatus of the invention constructed as described above, the controller is adapted to control the variable valve control system so as to control the exhaust gases emitted from the engine, such that the controlled exhaust gases permit the NOx catalyst to be recovered into an optimum state for reducing or removing the selected gaseous components in the exhaust gases that are emitted while the selected gaseous components should be removed.
The valve heads of the intake and exhaust valves in the closed states partially define the cylinders (combustion chambers) of the engine, and therefore the opening and closing timings and the amounts of lift of these valves will directly affect the condition of gases within the cylinders and the condition of exhaust gases to be emitted from the cylinders.
Accordingly, the exhaust gases emitted from the engine can be controlled in a short time by controlling the variable valve control system such that the controlled exhaust gases permit the NOx catalyst to be recovered into the state suitable for reducing or removing the selected components contained in the exhaust gases. Thus, the operation of the controller for recovering the NOx catalyst does not require an intolerably long time.
In particular, the time required for operating the internal combustion engine with a rich or stoichiometric air/fuel ratio for the purpose of removing the selected gas component(s) from the NOx catalyst can be shortened, and therefore the amount of fuel injected can be minimized, while at the same time suppressing deterioration of vehicle driveability.
Examples of the selected components to be reduced or removed from the NOx catalyst include nitrogen oxides (NOx) and sulfur oxides (SOx).
When the nitrogen oxides (NOx) are to be removed from the NOx catalyst, at least one reducing agent for reducing the nitrogen oxides is required. To this end, the controller may be adapted to control the variable valve control system such that the exhaust gases emitted form the internal combustion engine contain a large amount of reducing agent or agents.
The amount of the reducing agents contained in the exhaust gases can be increased, for example, by operating at least one of the cylinders of the engine with a fuel-rich air-fuel mixture having an air/fuel ratio lower than the stoichiometric value, so that the exhaust gases are fuel-rich, having a relatively low air/fuel ratio.
When at least one of the cylinders of the internal combustion engine is operated with a fuel-rich air-fuel mixture, the controller, such as an ECU, may be adapted to control the variable valve control system so as to shorten the period of opening of the intake valve of each cylinder in question, for example, so that the quantity of the air to be drawn into the cylinder is reduced to lower the air/fuel ratio of the air-fuel mixture to be burned in the cylinder. To further increase the amount of the reducing agents to be contained in the exhaust gases, the controller controls the fuel injection valve so as to increase the amount of the fuel to be injected into the cylinder in question, as well as controlling the variable valve control system so as to shorten the period of opening of the intake valve.
Although hydrocarbon (HC) is generally a typical reducing agent for reducing nitrogen oxides (NOx), hydrogen (H2) and carbon monoxide (CO) which have a higher reducing ability than hydrocarbon (HC) may also be used as the reducing agents. Hydrogen (H2) is produced in the combustion process of the air-fuel mixture, and carbon monoxide (CO) is produced by oxidization of hydrocarbon (HC). In view of this fact, the controller may be adapted to control the variable valve control system such that the exhaust gases emitted from the engine contain a large amount of hydrogen (H2) and carbon monoxide (CO).
For instance, the amount of hydrogen (H2) to be contained in the exhaust gases may be increased by advancing the moment of opening of the exhaust valve to a point of time before the moment of initiation of the exhaust stroke of the piston, that is, to a point of time in the expansion stroke of the piston, so that the air-fuel mixture being burned is expelled from the cylinder as part of the exhaust gases.
The amount of carbon monoxide (CO) to be contained in the exhaust gases may be increased, for instance, by retarding the moment of opening of the exhaust valve to a point of time after the moment of initiation of the exhaust stroke of the piston, so that the gases within the cylinder are sufficiently oxidized before they are exhausted from the cylinder.
When sulfur oxides (SOx) are to be removed from the NOx catalyst, in particular, when SOx poisoning of the NOx catalyst is to be eliminated, the NOx catalyst need to be exposed to a hot fuel-rich atmosphere. To this end, the controller may be adapted to control the variable valve control system such that the exhaust gases emitted form the engine are hot and relatively fuel-rich.
The temperature of the exhaust gases may be raised, for example, by advancing the opening timing of the exhaust valve of the cylinder in question so that the gases which have been just burned in the cylinder are discharged from the cylinder. The air/fuel ratio of the exhaust gases can be reduced by shortening the period of opening of the intake valve, by retarding the moment of opening of the intake valve and/or advancing the moment of closing of the intake valve, or alternatively by controlling the fuel injector valve so as to increase the amount of fuel injected into the cylinder.
In the case where the internal combustion engine according to the invention is equipped with spark plugs, the temperature of the burned gases upon opening of the exhaust valve of the cylinder in question can be further raised by advancing the moment of opening of the exhaust valve and retarding the ignition timing of the corresponding spark plug to retard the moment of combustion of the air-fuel mixture.
The NOx catalyst of the internal combustion engine of the invention may be, for instance, an NOx catalyst of an occlusion-reduction type adapted to occlude or absorb nitrogen oxides (NOx) contained in fuel-lean exhaust gases to which the NOx catalyst is exposed, and to release and reduce the occluded nitrogen oxides (NOx) when the exhaust gases are stoichiometric or fuel-rich. Alternatively, the NOx catalyst may be of a selective reduction type adapted to reduce or decompose nitrogen oxides (NOx) contained in the exhaust gases when the exhaust gases are fuel-lean and contain a reducing agent or agents.
In the internal combustion engine of the invention, the variable valve control system is preferably controlled so as to prevent a variation in the drive torque produced by the engine, while controlling the exhaust gases such that the controlled exhaust gases permit the NOx catalyst to be recovered into its optimum state for reducing or removing the selected gaseous components.
In the above instance, the selected components can be removed with a high degree of control response, while minimizing the drive torque variation of the internal combustion engine.
Another aspect of the invention provides an apparatus comprising: (a) an internal combustion engine of a lean-burn type capable of burning an oxygen-rich air-fuel mixture, which engine includes an intake valve and an exhaust valve for each of a plurality of cylinders thereof, (b) an NOx catalyst disposed in an exhaust passage of the internal combustion engine, (c) a variable valve control system capable of changing the opening and closing timing of the exhaust valve for said each cylinder of the engine, and (d) a controller configured to control the variable valve control system so as to placed the exhaust valve in an open position before a moment of initiation of an exhaust stroke of a corresponding one of the cylinders, when an exhaust gas emitted from the corresponding cylinder is desired to contain a reducing component for reducing NOx.
Preferably, the exhaust valve is opened after a moment of injection of a fuel into the corresponding cylinder.
By opening the exhaust valve within the period between the moment of the fuel injection and the moment of initiation of the exhaust stroke, a portion of the fuel injected into a combustion chamber of the engine can be introduced into the lean-burn NOx catalyst before that portion of the fuel has been burned within the combustion chamber. Since the above control is executed immediately after the controller determines that the learn-burn NOx catalyst requires a reducing agent or agents, the purification efficiency of the catalyst can be improved while minimizing otherwise possible deterioration of the fuel economy.
In the internal combustion engine including the variable valve control system according to the second aspect of the instant invention, the controller is preferably operated to control the variable valve timing control mechanism such that the exhaust valve is held open during at least a period between the moment of the fuel injection and the moment of ignition of the air-fuel mixture.
However, the exhaust valve may be opened prior to the moment of initiation of the fuel injection, and may be closed during the fuel injection. Further, the exhaust valve may be opened only once or two or more times in one cycle of operation of the engine.
When the internal combustion engine has a plurality of cylinders, the above-described control operation to open the exhaust valve at an appropriate time other than during the exhaust stroke may be performed with respect to all of the cylinders or a selected one or ones of the cylinders. When the engine has a plurality of exhaust valves for one cylinder, the above-described control operation may be performed with respect to all of the exhaust valves of the cylinder or for only a selected one or ones of the exhaust valves.
The variable valve control system provided in the internal combustion engine according to the first or second aspect of this invention may include an electromagnetic drive mechanism operable to generate electromagnetic force by application of electric current thereto, for bidirectionally moving the exhaust valves and/or the intake valves. Alternatively, the variable valve control system may include a hydraulic drive mechanism operated with a pressurized fluid for bidirectional movements of the exhaust valves and/or the intake valves.
In the second aspect of the present invention, the learn-burn NOx catalyst may be of the occlusion-reduction type or the selective reduction type.
The selective reduction type of lean-burn NOx catalyst is interpreted to mean a catalyst adapted to reduce or decompose NOx in exhaust gases emitted as a result of combustion of a fuel-lean air-fuel mixture, more specifically, to reduce or decompose NOx contained in an oxygen-rich atmosphere, in the presence of hydrocarbon. For instance, the lean-burn NOx catalyst may be a catalyst wherein zeolite carries ion-exchanged copper (Cu) or other transition metal, or zeolite or alumina carries a noble metal.
It is to be understood that the invention is applicable to a lean-burn internal combustion engine, such as diesel engines and lean-burn gasoline engines.